One of the most successful areas of synthetic biology is modifying microorganisms for applications in biotechnology. However, these engineered microbes could pose a risk if released into open environments. To overcome this, Chan et al. engineered two modular gene circuits into the bacteria Escherichia coli that act as kill switches. The “Deadman” circuit requires a specific small molecule to prevent a toxin from being expressed and killing the cell. In the “Passcode” circuit, small molecules must induce two transcription factors to inhibit toxin expression. An advantage over other biocontainment systems is that these circuits can be reprogrammed to respond to different inputs or to induce different killing mechanisms.